The present invention relates to a negative ion source. It is advantageously applied in the production of high-energy neutral atom beams used e.g. as an effective means for heating to thermonuclear temperatures plasmas produced in magnetic confinement fusion devices.
The atom beams used up to the present have had an energy below 80 keV/nucleon and are produced by accelerating positive ions up to said energy and these ions are then neutralized by collision with the molecules of a gas. However, the efficiency of the neutralization of positive ions drops when the energy thereof is increased.
The heating of plasmas in future nuclear fusion reactors requires atom beams having energy levels above 80 keV/nucleon. The good neutralization efficiency (&gt;60%) of H.sup.- and D.sup.- ions at these high energies makes them more interesting than positive ions for the production of atom beams with an energy above 80 keV/nucleon.
Hitherto, negative deuterium and hydrogen ion sources have been used in different particle accelerators and more particularly in the van de Graaf-tandem accelerator. The current intensities involved were limited to a few dozen microamperes. The methods for producing negative hydrogen or deuterium ions used up to the present in accelerators are as follows:
1. Double electron capture: a beam of positive ions of a few keV is partly converted into negative ions when it passes through an alkali metal vapour or gaseous target (E. B. Hooper, P. Poulsen and P. A. Pincosy, J. Appl. Phys. 52, 7027 (1981). PA1 2. Surface sources: The positive deuterium atoms of a plasma are accelerated to a cesium surface on which they are converted into negative ions (K. Prelec, dans Proc. of the Second Int. Symp. on the Production and Neutralization of Negative Hydrogen Ions and Beams, Oct. 6-10, 1980, BNL Report 51304, 1981, p. 145). PA1 3. Volume sources: Negative ions are extracted from reflex discharges (PIG sources) and peripheral areas of duoplasmatrons (K. Prelec et Th. Sluters, Rev. Sci. Instrum., 44, 1451 (1973). PA1 a magnetic multicusp confinement configuration realised by permanent magnets surrounding the plasma, PA1 thermionic electron emitters producing primary electrons made energetic by a potential difference applied between the thermoionic emitters and the generator anode and sufficient for fulfilling two functions: namely ionizing the gas or vapour for forming the plasma and exciting the molecules of the gas to high vibrational levels, whilst in this way aiding the production of negative ions by dissociative attachment of the slow electrons of the plasma to the excited molecules and also having: PA1 a negative ion extraction system, wherein: PA1 the number and distribution of the permanent magnets forming the magnetic multicusp confinement is chosen in such a way that the confinement time of the primary electrons is between 10.sup.-7 and 10.sup.-6 second, and wherein PA1 the thermoionic electron emitters are located in the multicusp magnetic field between the "saddle point" of the magnetic field, formed by two adjacent permanent magnets, and the centre of the plasma, in the vicinity of said "necks".
A large amount of research has dealt with the production of intense sources based on the first two methods. However, the production of the cesium or sodium vapour target, or the cesium surface are linked with very complex problems. Moreover, there is a certain pollution of fusion devices by alkali metals.
The recent discovery of a high production of H.sup.- or D.sup.- negative ions by dissociative detachment to H.sub.2 or D.sub.2 molecules with a high vibrational excitation and the observation of high negative ion densities in hydrogen and higher deuterium plasmas (M. Bacal, G. W. Hamilton, Phys. Rev. Letters, 42, 1958 (1979)) have led to the suggestion of also extracting H.sup.- or D.sup.- negative ions from multicusp-type plasma generators developed with a view to the production of positive ion beams.
The conventional magnetic multicusp plasma generator used as a positive ion source is characterized by the presence over the entire surface of the plasma, with the exception of the extraction surface, of a multicusp magnetic field produced by permanent magnets confining within the plasma, the high-energy primary electrons emitted by hot filaments made from refractory metals, e.g. tungsten. The conventional magnetic multicusp generator is also characterized by the position of the filaments, which must be located within the plasma and outside the multipolar magnetic field. All these features are quantitatively characterized by the confinement time of the primary electrons, which, in positive ion sources, is made as long as possible.
The negative ion current extracted from a conventional magnetic multicusp plasma generator is small compared with the positive ion current (0.8%).
K. N. Leung, K. W. Ehlers and M. Bacal have proposed (cf Rev. Sci. Instrum., 54, 56, 1983) the introduction into the multicusp of a "magnetic filter" for increasing the extracted negative ion current. The "magnetic filter" consists of a system of permanent magnet bars, each inserted into a copper tube, through which water is circulated to prevent the heating of the magnets which lead, to their demagnetization. The maximum current density of the negative ions extracted from this plasma generator is limited to densities of approximately 0.1 mA/cm.sup.2 /A of discharge current.